Milky Way

What is Milky Way?

Galaxies· Sun ~26,000 ly from the centre. Our home galaxy. Hundreds of billions of stars form a disc 100,000 light-years wide, and the band of the Milky Way in the night sky is that disc seen edge-on from within. The Sun sits in a spiral arm about 26,000 light-years from the centre; compare its scale with other galaxies on the size ladder.

Distance and scale

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Frequently asked

How big is Milky Way?

~100,000 ly.

In one line, what is Milky Way?

Galaxies — Our home galaxy.

What can I see of Milky Way here?

You can see the figures and story on this page.

The galactic year — the Sun's 230-million-year orbit

The Sun circles the galactic centre at 230 km/s, completing one orbit in about 230 million years — a galactic year. It has made about twenty such orbits since it was born, and during the last one dinosaurs appeared and vanished on Earth and humans arose. The Sun now sits on the inner edge of the Orion Arm, some 50 light-years above the galactic plane, bobbing up and down through the disk on a 60-million-year cycle.

Galactic rotation is not Keplerian, with inner orbits faster; the rotation speed stays nearly constant far from the centre. This "flat rotation curve" cannot be explained by visible stars and gas and became the first evidence for dark matter. Set the 3-D interstellar view to a 40,000 light-year half-width and the Sun's position is marked, giving a sense of where on this vast disk we are turning.

The galaxy in depth

The Milky Way is a barred spiral galaxy roughly 100,000 light-years across. A bar-shaped bulge crowded with stars sits at its heart, spiral arms wind outward from it, and a vast halo of dark matter envelops the whole disc. It holds somewhere between 100 and 400 billion stars, with the 4.3-million-solar-mass black hole Sagittarius A* at the centre. The Sun lies in the Orion Arm about 26,000 light-years out, circling the galaxy at 230 km/s once every 230 million years or so.

Because we live inside it, no photograph shows the whole galaxy from outside. The hazy band across the night sky is the disc seen edge-on; Galileo's telescope first resolved it into countless stars in 1610. Until Hubble proved in the 1920s that Andromeda was a separate galaxy, people assumed the Milky Way was the entire universe. The brightest, widest stretch of the band lies toward Sagittarius — the direction of the galactic centre.

The Sky view paints the Milky Way band into the background, so turn toward Sagittarius on a summer night to find the dense star clouds of the central region. On the size ladder, set the Milky Way beside Andromeda or IC 1101 to sense what 'a large galaxy' means on a cosmic scale.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.